What is the combustion process? (with photos)

Vehicle engines use the combustion process, burning fuel.

Simply put, combustion means burning. For the combustion process to take place, fuel, oxygen and an ignition heat source are needed to start a chemical chain reaction; in a fire, for example, wood is the fuel, the surrounding air provides the oxygen, and a match or lighter can ignite a fire. Increasing any of these elements will increase the intensity of the fire, while removing any of them will stop the process. If the fire is smothered with water or dirt, for example, oxygen can no longer reach the heat and fuel and it goes out.

Fuel

In a residential fire, firefighters use water or foam to put out the fire.

Fuel is the substance that burns during the combustion process. All fuels contain chemical potential energy; this is the amount of energy that will be released during a chemical reaction. The amount of energy a substance gives off when it burns is called the heat of combustion. Each fuel has a specific energy density, that is, how many megajoules (MJs) of energy are produced per kilogram (kg) of the substance; methane, for example, has an energy density of 55.5 MJ/kg, meaning it can deliver more energy than sulfur at 9.16 MJ/kg.

When used on an outdoor grill, propane joins with oxygen to combust.

A wide variety of substances can be used as fuel, but hydrocarbons are some of the most common. This includes methane, propane, gasoline, and jet fuel, to name just a few; all fossil fuels, including coal and natural gas, are hydrocarbons. Other substances commonly used as fuels include hydrogen, alcohol, and biofuels such as wood.

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During combustion, the fuel is transformed into heat and exhaust. When gasoline burns, for example, it produces water (steam), carbon dioxide, nitrogen, carbon monoxide, and other elements. Burning can also release particulates, which are tiny particles that float in the air; those released by burning fossil fuels and wood often contribute to air pollution. The exhaust can be used for beneficial purposes, such as providing the thrust that pushes a rocket through the air. Most exhaust is in the form of gas due to the heat that the combustion process produces, but it can also be in liquid or solid form.

Oxygen

Matches use friction to start the combustion process.

In order for fuel to be burned in the combustion process, it must also have oxygen. The most common source is air, which contains about 21% oxygen. Other sources, often known as oxidants or oxidizing agents, include hydrogen peroxide, potassium nitrate, and many more. When an oxidizing agent is introduced into a fuel, it releases oxygen and can increase the rate at which the fire burns.

Clearing bushes and dead vegetation from an area is more important to stopping a wildfire than water.

Like fuel, oxygen does not need to be in the form of a gas, although this is very common. In a solid rocket, for example, a solid oxidizer is mixed with the fuel to create the propellant, which burns when ignited and propels the rocket forward. The Space Shuttle and other spacecraft use liquid oxygen as part of the combustion process.

When a fire doesn’t have enough oxygen, it doesn’t burn out completely. This incomplete combustion produces carbon monoxide, carbon (soot) and other particles that contaminate the air. Incomplete combustion in a home fireplace or oven can release toxic fumes and be very dangerous.

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Warm

Problems with the combustion process of a natural gas appliance can lead to carbon monoxide poisoning.

Heat or ignition is what starts the combustion process. Since heat is also produced when something burns, once the process begins, additional heat is not always needed to keep the chemical chain reaction going. The initial spark that triggers the chemical process can be provided by a flame, friction, or even the heat of the sun.

In cases of spontaneous combustion, fermentation or oxidation can create enough heat to start a fire. In a compost pile, for example, bacteria can begin to break down the organic compounds, creating enough heat and oxygen to cause combustion. Some materials – called pyrophoric substances – ignite when exposed to air or water; phosphorus and plutonium are two examples. When these materials find a source of fuel, they can start a fire that is very difficult to put out.

Controlling the Combustion Process

As all three parts are required for combustion, increasing or decreasing any of them will affect the process. Increasing the amount of oxygen added to a fire by using an oxidizing agent, for example, will make the fire burn faster. Removing or reducing the fuel source will make it burn smaller or die out.

There are three basic ways to stop the combustion process:

take away the fuel, remove the oxygen, and/or take away the heat.

Combustion may also be stopped by stopping the chemical chain reaction that creates flames. This is especially important when certain metals — like magnesium — burn because adding water to the fire will only make it stronger. In such cases, dry chemicals or halomethanes are used to stop the reaction.

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Which of these is the best way to stop a fire depends on the type and the size of the fire. In a house fire, for example, firefighters use water or foam to stop oxygen from getting to the fuel and to reduce the temperature. While water can be used on a forest or wildfire, removing new fuel for the fire by clearing brush and dead vegetation from the area is often an important part of stopping it.

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